An Improved Method for Determination of Fumigant Degradation Half‐Life in Soil

Core Ideas Degradation of fumigants is an important component of their fate and transport. Traditional measurement of half‐life is limited by the large headspace in an incubation vial. A novel approach is presented that eliminates the incubation vial headspace. This novel approach yields shorter fumigant half‐lives due to greater contact with soil. Using the current approach, measurement of fumigant degradation half‐lives under realistic soil conditions is problematic due to the large headspace that is necessary above the soil during incubation. This results in a poor degree of contact between the fumigant and the soil's degrading surfaces, which differs markedly from field conditions. We report a simple, novel approach for determining fumigant degradation half‐lives in which the fumigant is in complete contact with the soil during incubation (no headspace) and which uses gastight, nonsorbing equipment commonly found in pesticide research laboratories. The approach was tested on a sandy loam soil. At both 23 and 50°C, the novel approach yielded much lower degradation half‐lives for cis ‐ and trans ‐1,3‐dichloropropene (1,3‐D) isomers and chloropicrin (CP) than the traditional approach. Percentage reductions ranged from 22 to 64% and were greater at the higher temperature. The results suggest that literature‐reported half‐lives of soil fumigants, and consequently their modeled soil–air emission rates, may be overestimated. The novel approach could potentially be applied to determining the degradation kinetics of other classes of volatile chemicals in soil.